1. Field of the Invention
The present invention relates generally to an HSDPA (High Speed Data Packet Access) communication system, and in particular, to a method of transmitting/receiving information about OVSF (Orthogonal Variable Spreading Factor) codes assigned to user data.
2. Description of the Related Art
HSDPA is a generic term that refers to data transmission schemes that bring high-speed data delivery to terminals by means of the HS-DSCH (High Speed-Downlink Shared Channel) and its related control channels in UMTS (Universal Mobile Telecommunications System). To support HSDPA, AMC (Adaptive Modulation and Coding) scheme, HARQ (Hybrid Automatic Retransmission Request) scheme, and FCS (Fast Cell Selection) have been proposed.
A. AMC (Adaptive Modulation and Coding)
AMC is a scheme for adapting the modulation and coding format based on a received signal quality of a UE (User Equipment) and a channel condition between a particular Node B and the UE to increase a use efficiency of an entire cell. Therefore, the AMC scheme involves a plurality of MCSs (modulation and coding schemes). The MCSs can be defined from level 1 to level n. In other words, the AMC scheme is an adaptive selection of an MCS level according to the channel condition between the UE and the serving Node B.
B. FCS (Fast Cell Selection)
When the UE enters a soft handover region, it selects the cell that is best able to transmit the required data. When a UE supporting HSDPA enters a soft handover region defined as the overlapped region of a first Node B and a second Node B, it establishes radio links with the Node Bs. The cells of the Node Bs that have radio links with the UE are the active set of the UE. Data delivery from only the best cell in a channel condition in the active set is FCS. Here, the best cell is a cell that has the best channel condition among the cells in the active set. The UE periodically monitors the channel conditions with the cells in the active set to check whether there is a cell better than the present best cell. If such a cell is detected, the UE transmits a Best Cell Indicator (BCI) to the cells in the active set to change the best cell. The BCI contains the identification (ID) of the new best cell. Upon receipt of the BCI, the cells determine whether the BCI indicates them. Then, the new best cell transmits an HSDPA packet to the UE on an HS-DSCH, thus reducing the overall interference.
C. N-channel SAW HARQ (N-channel Stop and Wait Hybrid Automatic Retransmission Request) Scheme
Two schemes are introduced to increase typical ARQ (Automatic Retransmission Request) efficiency. That is, a retransmission request and a response for the retransmission request are exchanged between a UE and a Node B, and defective data is temporarily stored and combined with corresponding retransmitted data. n-channel SAW HARQ scheme has been introduced to HSDPA to make up for the weak points in the conventional SAW ARQ scheme. In the SAW ARQ scheme, the next packet data is not transmitted until an ACK (Acknowledgement) signal for the previous transmitted packet data is received. Thus although the packet data can be transmitted, it is delayed to await the ACK signal. On the other hand, packet data can be successively transmitted without receiving the ACK signal for the previous packet data in the n-channel SAW HARQ, thereby increasing the use efficiency of channels. If n logical channels are established between a UE and a Node B, and identified by time or channel numbers, the UE, upon receipt of packet data at a certain timing point, can determine the logical channel that transmitted the packet data. Thus the UE can rearrange packet data in the right reception order or soft-combine the packet data.
A plurality of UEs share part of downlink transmission resources in an HSDPA communication system. The downlink transmission resources include transmission power and OVSF codes. Use of 10 OVSF codes when SF (Spreading Factor)=16 and use of 20 OVSF codes when SF=32 in the HSDPA communication system are under discussion.
A plurality of UEs can share a plurality of available OVSF codes at the same time, that is, which implies that OVSF code multiplexing is possible for the UEs at a certain time in the HSDPA communication system. OVSF code multiplexing will be described with reference to FIG. 1.
FIG. 1 illustrates an example of OVSF code assignment in a typical HSDPA communication system when SF=16. Referring to FIG. 1, each OVSF code is expressed as C(i, j) according to its position in a code tree. In C(i, j), the variable i indicates the SF and the variable j is a sequence number indicating the position of the OVSF code from the leftmost end of the code tree. For example, C(16, 0) represents the first OVSF code with SF=16 counted from the left in the code tree. As illustrated, the 7th to 16th OVSF codes with SF=16, that is, 10 OVSF codes C(16, 6) to C(16, 15) are assigned. The 10 OVSF codes can be multiplexed for a plurality of UEs as illustrated in Table 1.
TABLE 1UETimet0t1t2AC(16, 6)~C(16, 7)C(16, 6)~C(16, 8) C(16, 6)~C(16, 10)B C(16, 8)~C(16, 10) C(16, 9)~C(16, 10)C(16, 11)~C(16, 14)CC(16, 11)~C(16, 15)C(16, 11)~C(16, 15)C(16, 15)
In Table 1, UEs A, B, and C perform code multiplexing on their respective assigned OVSF codes at timing points t0, t1, and t2. A Node B determines the number of OVSF codes and their positions in the code tree to be assigned to each UE according to the amount of user data for the each UE and the channel condition between the Node B and the each UE.
It has been proposed that the OVSF code information is delivered to each UE on a downlink control channel in the HSDPA communication system. First a channel structure in the HSDPA communication system will be described.
The HSDPA communication system has an HS-DSCH for transmitting downlink user data, a downlink control channel, and an uplink control channel. The HS-DSCH transmits the UEs user data using the OVSF codes assigned to the HSDPA communication system. To support an AMC scheme, a HARQ scheme, and an FCS scheme, control information must be exchanged between the Node B and the UEs via the downlink and uplink control channels.
The uplink control channel transmits a periodical CQI (Channel Quality Information), ACK (Acknowledgement)/NACK (Negative ACK) signals indicating whether received user data has an error or not, and a best cell information. The downlink control channel transmits a particular UE an HI (HS-DSCH Indicator) indicating that the UE will receive user data on the HS-DSCH, an MCS level to be used for the data transmission, and information about OVSF codes to be assigned.
FIG. 2 is a block diagram of a transmitter for transmitting information about OVSF codes assigned to user data in the typical HSDPA communication system. Referring to FIG. 2, the transmitter takes charge of user data transmission on the HS-DSCH and control information transmission on the downlink control channel in a Node B of the HSDPA communication system. The transmitter includes an AMC controller 201, a scheduler 202, a transmission buffer 203, a turbo encoder 204, a user data transmitter 205, a control information generator 206, a channel encoder 207, and a control data transmitter 208. The transmission buffer 203 buffers user data received from a higher layer and outputs the user data to the turbo encoder 204 under the control of the scheduler 202. The turbo encoder 204 turbo-encodes the user data under the control of the AMC controller 201. The AMC controller 201 determines an MCS level for the user data according to the channel condition between the Node B and a UE, and controls the turbo encoder 204 to encode the user data according to the MCS level. The user data transmitter 205 modulates the encoded user data according to the MCS level, channelizes the modulated data, and transmits the user data to the UE.
The scheduler 202 controls information about OVSF codes used for the channelization and determines a user data transmission time and OVSF codes to be used for the UE, taking into account the amount and types of user data for other UEs.
The control information generator 206 converts information about the determined MCS level received from the AMC controller 201 and the OVSF code information received from the scheduler 202 to a format suitable for a radio channel. If the control information is to be transmitted on a DPCCH (Dedicated Physical Control Channel), the control information generator 206 converts the control information to a DPCCH transmit format. The channel encoder 207 channel-encodes the control information received from the control information generator 206 with a channel encoding scheme. Here, the channel encoding scheme is convolutional coding or turbo coding. The control data transmitter 208 performs modulation and channelization on the encoded control information and transmits the control information to the UE on a radio link.
FIG. 3 is a block diagram of a receiver for receiving OVSF code information in the typical HSDPA communication system. Referring to FIG. 3, the receiver receives user data on the HS-DSCH and control information on a downlink control channel in the UE. The receiver is comprised of a control data receiver 301, a channel decoder 302, a control information interpreter 303, a user data receiver 304, a turbo decoder 305, and a reception buffer 306.
Upon receipt of data on a radio link, the data is fed to the control data receiver 301 and the user data receiver 304. The radio link is a channel predetermined between the Node B and the UE for transmitting downlink control information, for example, a DPCCH. The control data receiver 301 despreads and demodulates the received data.
The channel decoder 302 channel-decodes the signal received from the control data receiver 301 in correspondence to the channel encoding scheme used in the transmitter. The control information interpreter 303 interprets MCS level information and OVSF code information from the control data received from the channel decoder 302. The MCS level information is output to the user data receiver 304 and the turbo decoder 305, and the OVSF code information is output to the user data receiver 304. The user data receiver 304 despreads and demodulates the received data using the OVSF code information and the MCS level information.
The turbo decoder 305 turbo-decodes the signal received from the user data receiver 304 in correspondence to the turbo coding scheme used in the transmitter using the MCS level information. The reception buffer 306 buffers the turbo-decoded signal and delivers the buffered user data to a higher layer at a particular timing point under a predetermined control. Thus, the receiver receives user data from the Node B on the radio link using the OVSF code and MCS level information.
As described above, the transmitter must transmit information about OVSF codes assigned to user data so that the receiver can detect the user data using the OVSF code information in the HSDPA communication system. Therefore, an efficient way of transmitting OVSF code information so that the first OVSF code and the number of OVSF codes to be assigned to user data are notified to the UE is under consideration.
Taking the situation specified by Table 1 as an example, in order to transmit user data to a UE A using OVSF codes C(16, 5) and C(16, 6) at time t0, information about the OVSF codes must be transmitted to the UE A earlier than time t0. The OVSF code information may be constructed as illustrated in Table 2.
TABLE 2UETimet0t1t2AC(16, 6)~C(16, 7)C(16, 6)~C(16, 8) C(16, 6)~C(16, 10)SP: 0110SP: 0110SP: 0110NC: 0010NC: 0011NC: 0100B C(16, 8)~C(16, 10) C(16, 9)~C(16, 10)C(16, 11)~C(16, 14)SP: 1000SP: 1001SP: 1011NC: 0011NC: 0010NC: 0100CC(16, 11)~C(16, 15)C(16, 11)~C(16, 15)C(16, 15)SP: 1011SP: 1011SP: 1111NC: 0101NC: 0100NC: 0001
In Table 2, SP (Start Point) represents a starting point of OVSF code assigned to user data in an OVSF code tree. The left most OVSF code is expressed as 0000 and the right most OVSF code is expressed as 1111. NC (Number of Code) is the number of OVSF codes assigned to the user data, expressed as a binary number. When 10 OVSF codes with SF=16 are assigned to an HSDPA communication system, expression of the SP requires 4 bits and expression of the NC requires 4 bits. Thus OVSF code information is delivered in the remaining 8 bits.
To generalize the expression of the OVSF code information, the number of bits for representing the SP is R(log2n) when NH OVSF (Number of Codes for HSDPA) codes with SF=n are assigned to the HSDPA communication system. Here, R(x) is an integer equal to or greater than a real number x. If a plurality of OVSF codes are assigned to a UE, the OVSF codes are assumed to be successive on the OVSF code tree.
The OVSF code information expressed as the SP and NC is part of the control information that was described referring to FIGS. 2 and 3 as being transmitted from the Node B to the UE via the radio link. Since the OVSF code information is transmitted to the UE on a downlink control channel, for example, a DPCCH, each time user data is transmitted on the HS-DSCH, it is preferable to minimize the size of the OVSF code information. However, the OVSF code information expressed as an SP and an NC requires more bits than are actually needed.